1. Field of the Invention
The present invention relates to a data head offset detecting circuit for use in a magnetic disk unit and a magnetic disk unit using the data head offset detecting circuit, which uses servo information on servo and data surfaces to correct an offset of data heads in the magnetic disk unit.
In recent years as the magnetic disk unit has had an increasingly larger capacity, narrower track gaps have been utilized in magnetic disk units.
When the conventional head positioning control using the servo information on the servo surface alone is used with a magnetic disk having a higher track density with such narrower track gaps, the data head, which moves over the data surface and is controlled to be ontrack based on the servo information on the servo surface is likely to be offtrack and cannot read the data on the data surface if the operating ambient of the magnetic disk unit, especially, the ambient temperature, changes from low to high or vice versa.
A head positioning circuit for offset correction was proposed that corrects an offset by recording servo information on the data surface as well and using the servo information on both the servo and data surfaces to prevent the data head from being offset. This invention relates to the above circuit.
2. Description of the Related Art
For this offset correction, a head positioning control is done by effecting an offset detection at every predetermined time interval depending upon the change of ambient temperature, storing the offset thus detected into a memory for each head, and reading a corresponding offset at the time of data write or read to correct the offset.
On the assumption that the servo head is accurately controlled to be on track based on the servo information on the servo surface, this offset detection is done to read the servo information on the data surface by the data head and detect an offset of the data heads from the servo head.
During offset detection, however, the servo head under ontrack control is likely to be offtrack due to external vibration, poor accuracy of control or the like so that the offset of the data head is detected at the timing of the offtrack. No accurate correction of the data head offset can be done.
A magnetic disk unit may have, for example, four magnetic disks mounted on the rotating shaft of a spindle motor and thus rotated at a predetermined speed.
Of the four magnetic disks, the upper three have a data surface formed on either side, front and rear, thereof and the lower one has a data surface formed on the front side thereof and a servo surface on the rear side thereof.
Data heads are provided opposite the data surfaces of the magnetic disks and a servo head is provided opposite the servo surface.
The data heads and servo head are driven together by a voice coil motor (will be referred to as "VCM" hereafter) and move in a direction crossing the tracks on the magnetic disks.
The servo surface over which the servo head is positioned has servo information recorded in all the cylinders (cylinder: all tracks accessed by all the magnetic head when a comb is in a certain position) thereof, and a position signal indicative of the position of the servo head on the track is provided based on a signal read by the servo head.
On the other hand, the data surfaces of the data heads have recorded in the free space of a predetermined cylinder track or of all the cylinder sectors thereof servo information used to detect an offset of the data heads.
First, if the servo head and data head are in the cylinder center, no offset correction is necessary; actually, however, the data head is off the cylinder center on which the servo head is positioned due to the difference in the expansion coefficient between the metals used to form the head actuator and the like.
Under the ontrack control of the heads based on the servo information on the servo surface, the offset of the data head from the servo head is detected based on the servo information read by the data head from the data surface.
At the time of write or read, the data head can be accurately positioned in the cylinder center by displacing the heads (offset correction) to eliminate the offset.
However, the aforementioned conventional offset detecting circuit has a disadvantage in that if the data heads are oscillated due to a disturbance or poor accuracy of positioning control during detection of a data head offset, then detection of a data head offset done at the time of such oscillation will result in a measurement error.
The offset of the data head from the servo head should be detected, but the offset of the data head from the cylinder center is detected in a condition where the servo head is offset from the cylinder center because of the oscillation of the heads.
If an offset correction is done based on the offset thus detected, the data head will be offset from the cylinder center so that the offset correction will cause an offset of the data head.
The related arts regarding this invention are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-266781.